Wavelength division multiplexed (WDM) optical communication systems are known to include one or more photonic integrated circuits (PICs) in which multiple optical signals, each having a different wavelength or associated carrier frequency, are combined into a modulated output signal for transmission over an optical fiber. Such systems typically include transmitters having a laser supplying light at a carrier frequency, a modulator configured to modulate the light output of the laser, and an optical combiner to combine each of the modulated outputs into the modulated output signal. The carrier frequencies associated with each of the modulated outputs, collectively the frequencies defining a carrier frequency grid, or simply frequency grid, may be spectrally spaced from each other to define a channel spacing with respect to the grid. The WDM optical communication system may include multiple PIC devices, the outputs of each being spectrally combined or interleaved to form a combined output signal for transmission over an optical fiber. The frequency grids of each of such PIC devices may be spectrally spaced or offset from each other such that a portion of each frequency grid overlaps a portion of the remaining frequency grids. Thus, each adjacent signal channel may be from different wavelength grids, the adjacent signal channels defining a channel spacing with respect to the interleaved signal.
Historically, such modulated output signals were amplitude or intensity modulated. More recently, however, more advanced transmission systems, such as coherent systems, use more complex modulation formats. Such complex modulation formats may employ phase-shift keying (PSK), for example, which offer higher capacity than the intensity modulated signals. Some examples of modulation formats which incorporate PSK include binary phase shift keying (“BPSK”), quadrature phase shift keying (“QPSK”), differential phase-shift keying (“DPSK”), and polarization multiplexed differential phase-shift keying (PM-DPSK), to name a few.
As with other transmission systems, coherent transmission systems may provide for a data capacity over a corresponding link of a given distance or reach, the capacity provided within a margin, such as a bit error rate for example. However, links employed for transmission of optical signals may include impairments which may limit the performance of the transmission system. Such impairments may include, for example, various forms of dispersion due to random imperfections and asymmetries of the optical fiber associated with the link, such as polarization mode dispersion or cross phase modulation chromatic dispersion. For purposes herein, impairments shall include any linear or non-linear impairment which impacts the integrity of a transmitted optical signal. Such impairments may be associated with the optical signal itself, such as optical power or a modulation scheme employed to modulate the optical signal, or may be associated with structure of the transmission system, such as an optical fiber over which the optical signal is transmitted.
These impairments may limit the rate at which data is transmitted over the link and, ultimately, may require the data to be transmitted at a reduced data rate or with a modulation format which is more tolerant to one or more of the impairments present in the link. Accordingly, coherent transmission systems including a number of signal channels and employing fixed signal channel spacing may need to deploy less than the number of signal channels available due to impairments in the link and the signal channel spacing available. Such a reduction in deployed signal channels, e.g. less than the total number of signal channels available be utilized, may result in spectral gaps across the signal channels and an underutilized transmission system.
Alternatively, coherent transmission systems may be provided which include a number of signal channels and employ fixed signal channel spacing to achieve a maximum capacity. Such transmission systems may be designed taking into account impairments which may be present in a typical optical link for example. Accordingly, the fixed channel spacing may be selected to provide amble bandwidth to overcome a certain degree of impairments. However, if that degree of impairments is not observed in an optical link used for transmission, e.g. the total bandwidth provided by the fixed channel spacing is not fully utilized, spectral gaps may be present across the signal channels resulting in an underutilized transmission system.
What is needed is a coherent transmission system which provides variable channel spacing in order to maximize the capacity of the system in light of impairments which may be present. The coherent transmission system can preferably adjust the carrier frequencies of each individual signal channel of each frequency grid to achieve a desired channel spacing. The desired channel spacing may be consistent with a desired capacity or reach associated with a link, as part of the network infrastructure of the coherent transmission system for example. Also, what is needed is a coherent transmission system which can continuously adjust the channel spacing in response to a change in the transmission system. Further, what is needed is a coherent transmission system which can adjust the channel spacing based upon a modulation format or a change in the modulation format.